Change axis of Fork Extrude
Rigging is admittedly not my expertise, but if I understand correctly, this has to do with unapplied transforms, and the direction the model is facing. You have x axis mirror enabled, but are parallel to the x axis (model facing left ortho view) so are unable to properly mirror to get the expected results. 

I would first rotate your model 90 degrees on the Z axis to face front ortho view, and apply transforms with Ctrl+A and you should be able to proceed as desired. See .gif below for the steps: